WEDNESDAY, April 8, 2015 (HealthDay News) -- For patients undergoing heart surgery, using transfused red blood cells stored for 21 days or more is as good as using blood cells stored for 10 days or less, a new study finds.

Previous studies have suggested that transfusing red blood cells stored for more than two to three weeks was linked to serious, even deadly, results, the study authors noted.

But the new study results, reported April 9 in the New England Journal of Medicine, disprove the thinking that fresher blood is better for heart surgery patients, the researchers said.

"It does not appear necessary to restrict complex cardiac surgery patients to receive only red cells stored for shorter lengths of time, as receiving red blood cells of shorter storage duration does not confer any benefits for these patients," said lead researcher Dr. Marie Steiner. She is a pediatric hematologist-oncologist at the University of Minnesota Medical Center in Minneapolis.

Katz explained that red blood cells are stored and refrigerated in a solution that keeps them from clotting. They can be kept and used for a maximum of 42 days. As these cells age, they look different from fresher cells when examined under a microscope. "People have said, 'That can't be good,' " he said.

"While it seems obvious that funnier-looking cells from older bags wouldn't do as well, we now have major randomized trials that say that's not true," Katz added. "That just shows you that looking at a stored red cell under the microscope doesn't tell you what's going on."

For the study, Steiner and her colleagues randomly assigned nearly 1,100 patients undergoing heart operations to receive either red blood cells stored for 10 days or less, or stored for 21 days or more. The researchers looked for changes in what is called the Multiple Organ Dysfunction Score, which ranges from 0 to 24. The higher the score, the more severe the organ damage.

Patients were followed for seven days or until they died or left the hospital. The storage time of red blood cells given to those who received the freshest ones was seven days, compared to 28 days for those who received the oldest red blood cells.

The results: patients who got the newer red blood cells had Multiple Organ Dysfunction Scores that increased 8.5 points. This compared to an increase of 8.7 points for those who received the oldest red blood cells, the researchers found.

Over seven days, 2.8 percent of patients who received the freshest red blood cells died, compared with 2 percent of those given the oldest red blood cells. And at 28 days after their surgeries, 4.4 percent of those given the freshest red blood cells died, compared with 5.3 percent of those given the oldest red blood cells, the researchers said.

While side effects were similar between the groups, those given the older red blood cells were more likely to develop jaundice caused by the breakdown of older red cells -- a condition called hyperbilirubinemia, the investigators found.

"This study closes the door on an issue raised seven years ago in a study that suggested that there was increased risk to cardiac patients who received older blood versus fresher blood," said Dr. Richard Benjamin, chief medical officer of the American Red Cross.

"Hospitals had been asking, actually demanding, the freshest possible blood because they were saying older blood was doing harm. And that places incredible strain on the blood supply -- we simply can't supply all patients with the freshest possible blood," he said.

This study shows that blood -- whether seven days old or 28 days old, is equally good, Benjamin said. "You shouldn't be demanding the freshest blood as you have been for the last seven years," he said.